The invention concerns an NMR probe head for investigating a temperature-sensitive test object in a volume under investigation, comprising at least one RF receiver coil which is cooled to a cryogenic temperature during operation and is surrounded by a housing, wherein at least one heatable separating wall is provided between the RF receiver coil and the test object.
An NMR probe head of this type is disclosed in U.S. Pat. No. 5,694,775.
Magnetic resonance imaging utilizes RF receiver coils for exciting and receiving RF signals of the spins of a test object to be investigated. The inherent loss of small RF receiver coils plays a large role, whereas the loss in the test object, e.g. a patient or an animal, is secondary. In order to reduce the inherent loss of the RF receiver coils, the latter are cooled to cryogenic temperatures.
U.S. Pat. No. 5,913,888 discloses an NMR probe head having an RF receiver coil which is cooled using a pulse tube cooler and a non-metallic thermal contact body for transmitting refrigerating capacity of the pulse tube cooler to the RF receiver coil.
The manual “Animal support for BioSpec Systems—Operation Instructions and Specifications” of Bruker BioSpin MRI GmbH discloses an animal support 10 for use in NMR measurements on rodents. The conventional animal support has a tray-shaped structure with a chamber system through which heated water may be guided for controlling the temperature of the animal. The animal support also comprises a device for supplying anaesthetic or for artificial respiration of the animal. The temperature of the animal can be controlled via the animal support only from below and from the side of the animal, since the animal support is open at the top.
In order to obtain optimum signal quality, the RF receiver coils are disposed as closely as possible to the test object. Unless special measures are taken, thermal energy is transported from the separating wall to the cold RF receiver coil via black body radiation when the separations between the RF receiver coil and the separating wall are small. Since the temperature of the RF receiver coil is on the order of magnitude of 30K, and the temperature of the separating wall is equal to or slightly higher than room temperature, the thermal flow is generally very large and the separating wall is cooled in an undesired fashion. In particular, when the test objects are alive, this may have generally undesirable consequences (death of an animal, permanent damage due to undercooling, change of the molecular structure of the test object). To obtain optimum performance of the NMR probe head, the separating wall must be excellently thermally insulated from the cold RF receiver coil, and the separating wall must permit undamped passage of the RF signals (RF window).
This problem is solved in accordance with prior art in that heat is uniformly supplied along the surface of the separating wall. To this end, U.S. Pat. No. 5,694,775 proposes guiding of a liquid of a suitable temperature through a gap between the cooled RF receiver coil and the housing in order to keep the surface of the separating wall at the desired temperature.
Cooling of the test object is also conventionally prevented using a warm air flow. This may result in dry eyes, in particular, when living test objects are investigated.
DE 101 60 608 A1 discloses arrangement of a gas cushion between a separating wall of the housing and the measuring object, through which a gas of suitable temperature flows. The gap between the separating wall and the housing must thereby be sufficiently large for passage of the heating gas. This is, however, difficult to control and can produce undesired malfunction which may not be detected and cause local freezing of the test object. It is also very difficult to guide a temperature-controlled gas in a controlled and uniform fashion when the surfaces have complex curvatures.
The conventional devices pose the problem that the overall structure of the insulation need be sufficiently thick in order to prevent excessive cooling of the test object. On the other hand, the thickness of the separating wall including insulation must be sufficiently small so that the RF receiver coil is within an appropriate range within the signal source in the test object, in order to prevent unnecessary reduction of the signal/noise ratio. Due to the multi-layer structure of the conventional NMR probe heads and the required, relatively large thickness of the insulation layer and also of the gas- or liquid-carrying layer, the separations between the RF receiver coil and the test object are large (at least 2 mm). These separations are disproportionally large for applications with relatively small test objects (brain of a mouse) and greatly limit the signal/noise ratio that can be achieved.
Moreover, the fluids that are conventionally used for heating the space between the RF receiver coil and the test object may have a negative influence on the NMR measurement quality, since they are located close to the volume under investigation and for this reason, spins of the respective fluid could also be excited.
It is the underlying purpose of the invention to propose an NMR probe head with which the RF receiver coil may be disposed as closely as possible to the test object to be measured without inadvertently cooling it.